By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Greenlee County is in southeastern Arizona just west of the New Mexico State boundary. It was organized from part of Graham County in 1910. Copper is the metal of principal importance, but the county has also produced significant amounts of gold and silver.
The total gold production of the county from 1882 through 1959 was about 228,000 ounces, almost all of which was a byproduct from the copper ores of the Clifton-Morenci district, one of the most productive copper camps in Arizona. A small amount of gold was derived from the silver ores in the Ash Peak district.
Placer mining was attempted several times in the Clifton-Morenci district, but the results were discouraging. The total recorded placer gold output is about 1,000 ounces.
ASH PEAK DISTRICT
The Ash Peak district is 12 miles west of Duncan. Records indicate that the deposits were exploited as early as 1907, but only silver was produced during these early operations (Elsing and Heineman, 1936, p. 93; V. C. Heikes, in U.S. Geological Survey, 1907, p. 161). Extensive development work was done in 1918 (Lines, 1940, p. 3), but the results appear to have been discouraging. Mining was resumed from 1936 through 1954, resulting in the recovery of 11,296 ounces of gold. The district was again inactive from 1954 through 1959.
The bedrock of the district consists of a series of rhyolite and andesite flows and tuffs cut by numerous dikes and volcanic plugs of diabase. The ore occurs in a vein that follows a dike intruded along a fault. The ore bodies are fairly continuous and consistent in grade and contain argentite in banded chalcedonic quartz, and varying amounts of calcite, rhodochrosite, pyrite. Lead and copper minerals occur locally. Gold, lead, and copper were produced as byproducts from the silver ores (Lines, 1940, p. 3-4, 24).
The Clifton-Morenci district is in west-central Greenlee County near the towns of Clifton and Morenci.
The first ore discovery was made in 1872, but early development was hampered by lack of transportation and the activities of hostile Indians. The completion of the Southern Pacific Railroad in 1881 lowered transportation costs sufficiently to permit large-scale mining of the copper ores. The discovery in 1893 of large low-grade copper ores at Copper Mountain at Morenci assured a certain degree of stability and permanence to the future of the district (Lindgren, 1905, p. 33-34).
At first several companies were involved in development and mining, but after several mergers and consolidations, the Phelps Dodge Corp. was the major operator from 1921 through 1959. Total gold production from 1873 through 1959 was about 203,000 ounces; nearly all production was recovered as a byproduct of the copper ores.
The rocks of the district consist of Precambrian granite and schist unconformably overlain by an aggregate thickness of 1,000 feet of limestone, shale, and quartzite which ranges in age from Cambrian to Carboniferous. The Paleozoic rocks are unconformably overlain by shale and sandstone of Cretaceous age. The Cretaceous and older rocks are faulted and intruded by stocks, irregular masses, dikes, and sills of granitic, monzonitic, and dioritic porphyry.
The mineral deposits are probably genetically related to these intrusions. The sedimentary rocks and intrusive porphyries are capped by a series of lavas, tuffs, and breccias of Tertiary age. Overlapping all the preceding rocks are extensive deposits of Gila Conglomerate, of Tertiary and Quaternary age.
The ore deposits are of three general types: tabular bodies in limestone or shale near contacts of stocks or dikes, fissure veins, and irregular disseminations in porphyry, quartzite, or other rocks (Lindgren, 1905, p. 97-99). The most important of these are disseminated deposits which have been oxidized and enriched by the supergene copper sulfides.
These ore bodies are capped by an oxidized gossan of limonite, secondary quartz, and minor amounts of copper oxides and carbonates. The zone of secondary enrichment, below the oxidized zone, contains abundant chalcocite. The protore consists of pyrite, and small amounts of chalcopyrite and sphalerite (J. B. Tenney, in International Geological Congress, 1935, p. 218-221).
Second in importance are the tabular replacement deposits in limestone and shale. Ore bodies are in the oxidized parts of the deposits and consist of masses of malachite, azurite, and cuprite and small amounts of native copper and chalcocite in a gangue of decomposed lime silicate minerals.
The fissure vein deposits, which are nearly exhausted, are in fissures in Precambrian granite and Cambrian quartzite closely associated with dikes. The ore minerals are pyrite, chalcopyrite, sphalerite, molybdenite, chalcocite, cuprite, chrysocolla, brochantite, and malachite.
The ores of the district contain relatively insignificant quantities of gold, and the mode of occurrence and mineral associations of the gold are not clearly understood.